730 
C. GORDON HEWITT. 
tlie dorsal side was couvex and the ventral side flat. Linnfeus 
(1758) called this mite Acarus muse arum from de Geer’s 
description^ and Geoffrey (1764) found what appears to be 
the same, or an allied species of, mite, which he called the 
‘‘brown fly-mite.” Murray (1877) describes a form, Trom- 
bidium parasitic u m,^ which is a minute blood-red mite 
parasitic on the house-fly. He says : “ In this country they 
do not seem so prevalent, but Mr. Eiley mentions that in 
North America, in some seasons, scarcely a fly can be caught 
that is not infested with a number of them clinging tenaciously 
round the base of the wings.” As it only possessed six legs 
it was doubtless a larval form. 
Anyone who has collected Diptera as they have emerged 
from such breeding-places as hot-beds, rubbish and manure 
heaps will have noticed the frequently large number of these 
insects which are to be found carrying immatui’e forms of the 
Acari. These are being transported merely by the flies in 
the majority of cases. Mr. Michael tells me that he used to 
call such flies “the emigrant waggons ” — a very descriptive 
term. Many of these mites belong to the group Gamasidm — 
the super-family Gamasoidea of Banks (1905). These mites 
have usually a hard coriaceous integument. In shape they are 
flat and broad and have rather stout legs. Sometimes imma- 
ture forms of these mites swarm on flies emerging’ from rubbish 
heaps. Banks holds the opinion that they are not parasitic, 
but that the insect is only used as a means of transportation. 
It is difficult to decide whether this is so in all cases. I have 
illustrated (fig. 14) a specimen of the small house-fly, H. 
canicularis, caught in a room ; on the under-side of the fly’s 
abdomen a number of immature Gamasids" are attached, 
' This species was named Atoma parasiticnm and later Astonia 
parasiticnni l>y Latreille (‘ Magazin Encyclopedi(pie.' vol. iv, p. 15. 
17!15). Mr. A. D. Michael tells me that the. genus was founded on 
Tromhidium parasiticnm of de Geer. They were really larval 
Tromhidiida' and Atoma was founded on larval characters ; probably 
any larval Tromljidium came under the specific name. 
“ Being unable to identify these immature specimens I submitted 
them to Mr. Michael, who kindly informs me that it is extremely diffi- 
